Electromagnetic interference (EMI) or radio frequency (RF) interference from electrical devices, signal sources, etc., can interrupt or otherwise degrade the effective performance of certain telecommunication, computer, and other electrical/electronic equipment. As a result, filters are commonly used in numerous types of electronic circuits to allow signals of a desired frequency to pass along a signal path, for example, while reducing the propagation of other undesired frequencies.
One particular type of filter commonly used in electrical and electronic devices is the ferrite filter. Ferrite filters act as chokes which attenuate signals to a desired frequency range. One basic type of ferrite filter is the so-called ferrite “bead” which typically includes a single, “soft” ferrite material having a particular attenuation range that depends upon the type of ferrite used.
A somewhat more sophisticated ferrite shield is disclosed in U.S. Pat. No. 4,796,079 to Hettiger entitled “Chip Component Providing RF Suppression.” The ferrite shield is formed by surrounding an electrical conductor with ferrite material and is made in the shape of a leadless chip to permit placement on a circuit board by chip insertion machines. In one embodiment, different shield elements are disposed along the lateral conductor signal path, where each shield element includes a different type of ferrite material having different impedance-frequency characteristics. Each shield element thus contributes to the overall impedance-frequency characteristics of the shield.
EMI can be a particular problem between about 800 MHz and about 15 GHz, for example. However, soft ferrites based on a spinel crystal structure are not traditionally used over 1 GHz or so because the ferromagnetic frequency response thereof decays rapidly in this frequency range. More particularly, most prior art surface mount EMI filters, such as those noted above, typically use soft ferrites which exhibit an impedance maximum below 2 GHz. While other types of ferrite are used in microwave frequencies in devices such as monolithic microwave integrated circuits (MMICs) and in elements of radar devices that operate in the low to high GHz frequency range, such devices may not readily lend themselves to relatively small-scale applications.